A present mobile communication system has a plurality of base stations, and each base station has an area in which the base station can communicate with a mobile terminal. When a certain mobile terminal moves out of the area of the base station which is communicating with the mobile terminal, the mobile terminal changes the base station to other base station which has an area in which the base station can communicate with the mobile terminal. This is called handover or handoff.
In a mobile communication system, such as the PHS (Personal Handy-phone System), since a design for placement of base station is not performed when a base station is placed, a system in which the base station avoids interference autonomously and communicates is adopted. In this system, neither the base station nor a base station control apparatus grasps an absolute position of the mobile terminal because of its property. Therefore, with respect to the handover, the Japanese Laid-Open Patent Application No. Hei 5-14265 describes that a mobile terminal judges communication degradation with movement of the mobile terminal and then sends a connection call to other base station. That is, a mobile terminal initiative handover is adopted. The mobile terminal looks for a surrounding base station, and sends a handover request to other base station based on the search result. It is determined by condition or strength of a radio wave connecting the call between the base station and the mobile terminal, or remaining resources of the base station whether the handover is possible. For this reason, if the mobile terminal can not find the handover base station, the mobile terminal must send a handover request to other base stations repeatedly, and it may take long time from the handover request to a handover completion.
The handover depends greatly on performance of each mobile terminal, such as a receiving sensitivity and a function to look for the handover base station and to require the handover. For this reason, when the radio terminal moves at high-speed, a problem that the handover is overdue and the communication goes out occurs.